1. Technical Field of the Invention
This invention relates generally to the field of processing press formed paperboard products. Specifically, the invention is related to a press formed paperboard container which is useful for preparing a food product, for baking the food product at elevated temperatures and for serving the cooked food product.
2. Description of Related Art
The consuming public is increasingly interested in reducing the time required to prepare, cook and serve food items. This is equally true for food service establishments. One way of reducing this time is by providing disposable containers which are capable of holding food items during baking which can be thrown away after use. The container used for this purpose must be sufficiently rigid to support the food item contained therein and capable of holding any liquids produced during baking within the container. It must be fabricated from a disposable material which is capable of withstanding high temperatures required during baking.
Formed fiber containers, such as paper plates and trays, are commonly produced either by molding fibers from pulp slurries or by press forming flat paperboard blanks between dies into the desired shape. Prior to recent developments, molded fiber paper products were considered to exhibit the greatest resistance to bending and were believed to relatively strong. However, molded fiber products generally have rough surface characteristics making it very difficult to apply a surface coating. Without such a surface coating, such a container is susceptible to penetration by water, oil and other liquids.
Press formed containers, on the other hand, generally exhibit rather smooth surface characteristics, so that surface decorations or surface coatings can be readily applied to their surface, even prior to pressing. Initially, however, due to unsophisticated press forming techniques, pressed containers were not capable of maintaining their original pressed shape, especially if angled side walls or rims were desired, because pleats were forced into the sides of the formed container. The pleats created areas of weakness which caused these containers to lose their shape.
Recent developments in press forming processes have greatly increased their rigidity and their ability to maintain their shape. U.S. Pat. No. 4,609,140, issued to Van Handel et al. and assigned to James River --Dixie Northern, Inc., is directed to a rigid paperboard container and method and apparatus for producing the same. Note also, U.S. Pat. No. 4,721,500, which is a divisional of the application of 4,609,140. Specifically, this process provides a press formed paperboard container which includes a bottom wall, an upturned side wall and an overturned rim extending from the side wall that is denser and thinner than the rest of the container. The container is formed by pressing a flat circular blank between upper and lower die members to shape the container in proper form. The surface of the die members are designed to exert extremely high compressible stresses on the rim portion, particularly at scored or folded areas formed in the rim. The high compressive forces, along with proper moisture levels in the paperboard and the heating of the die members, combine to cause the paperboard blank to deform plastically and densify the rim portion to create a rigid container. The densified rim portion is rigid in structure to provide a sufficient resistance to bending for the entire container.
Other means of producing rigid press formed paperboard containers have also been developed. U.S. Pat. No. 4,606,496, issued to Marx et al., and assigned to James River Corporation of Virginia, is directed to a rigid paperboard container which is integrally formed from a substantially homogeneous paperboard blank by a press. The container includes a bottom wall, an upwardly extending side wall, a first curved portion joining the side wall to the bottom wall, an outwardly extending rim, a second curved portion joining the rim to the side wall and a downwardly extending lip. Further, the container includes a plurality of densified areas, similar to those provided in the Van Handel et al. patents, that radially extend through and are circumferentially spaced about the side wall, the second curved portion and the rim portion. However, these densified regions are formed from pleats which include at least three layers of paperboard created during press forming by applying sufficiently high pressure to reform the layers into a cohesive, fibrous structure. Therefore, necessary pleats, which had previously been areas of weakness, can now be replaced by areas having increased density compared to the remainder of the container and comprising at least three layers of fibrous paperboard material. This increases the rigidity of the container and allows circular press formed containers to exhibit the smooth surface benefits of previous press formed containers, while also providing the rigidity of a molded paper container. See, also, U.S. Pat. No. 4,721,499, a divisional of the above-noted reference, which is directed to the method of producing the rigid container described above and U.S. Pat. No. 4,832,676, issued to Johns et al., which is directed to a method and apparatus for forming paperboard containers.
A second desirable characteristic for a paperboard container useful for holding a food product is a resistance to liquids. This is especially true for containers which are used for heating or baking the food item contained therein. U.S. Pat. No. 4,026,458, issued to Morris et al., is directed to a deep drawn paperboard container which includes an inner water and grease impermeable layer coated on the inner wall of the container. Preferably, the inner layer is polyethylene, which allows the food item held by the container to be cooked at temperatures of less than about 350.degree. F. Because the inner, liquid impermeable layer is polymeric in nature, the container is not suited for baking foods, such as pizza, which require temperatures of approximately 450.degree. F. for proper baking. In addition, the temperature of the mandrels used for shaping the container must generally be held at different temperatures, so that the inner coated layer will not stick to the male mandrel.
Although the container set forth in Morris et al. is deep drawn, the blank which is shaped into the container merely includes score lines, located at the corners, which form pleats after the blank is subjected to forming mandrels. This formation does not provide the superior rigidity of the container set forth in the Marx et al. patents because it does not include the downturned lip portion and the radially extending densified regions which include at least three layers. Note, also, U.S. Pat. No. 4,381,278 to Ingraffea also discloses a polymeric coated, deep drawn paperboard container that suffers from the same limitations as the container of Morris et al.
A second problem with polymer coated containers, as set forth in Morris et al. and Ingraffea, is the public's increasing concern with the environment. Polymer coated containers are not sufficiently biodegradable to satisfy the environment conscious public. Paper alone would be a preferred container, but uncoated paper does not exhibit a sufficient degree of resistance to liquid penetration which is required for a container used for baking a food item.
Parchment paper can inherently act as a barrier layer against oil and grease for containers. U.S. Pat. No. 1,633,787, issued to Kress, discloses a container which includes at least one layer of "vegetable parchment" to render the container oil and grease proof. Further, U.S. Pat. No. 4,014,496, issued to Christensson, is directed to a cup formed container, which may include a lining blank of paper, waxed paper, or parchment. The lining blank can be glued or laminated over the main part of the paperboard blank that is ultimately shaped into a container. However, as set forth at col. 5, lines 50-55, lined containers may cause problems. Specifically, the lining cannot be laminated over the complete cardboard container blank to allow proper joining of the joint flaps of the blank.
In addition, the parchment lined containers, set forth above, are not contemplated as useful containers for baking food products. Although glue-tapered trays, similar to Christensson which include an inner lining of parchment paper, have been used to cook food products, such as fish fillets, the containers are not designed to be completely leak proof due to the glued flaps of the joined container. Therefore, such a container could not be used to serve a cooked food product because of the possibility of leakage. Moreover, glue-tapered trays or containers are not as dimensionally stable as press formed containers, so users of such a container for baking food items run the risk of the container disassembling during baking.
Consequently, there is a need for a paperboard container for baking a food product which is substantially rigid, so that it will maintain its dimensional integrity during baking, and which is lined with a grease and oil impermeable layer capable of withstanding elevated baking temperatures exceeding 350.degree. F. The prior art has failed to provide such a container.